Magnetic resonance (MR) imagers or scanners have been developed that produce images for diagnosing disease and contrasting healthy tissue from abnormal tissue. An MR imager or scanner typically employs a superconducting magnet to generate the large magnetic fields which it requires for operation. To realize superconductivity, a magnet is maintained in a cryogenic environment at a temperature near absolute zero. Typically, the magnet includes one or more electrically conductive coils which are disposed in a cryostat and through which an electrical current circulates to create the magnetic field.
Meanwhile, radiation therapy has been developed which can focus a radiation beam (radiotherapy beam) on a target region of interest in a patient and preferentially destroy diseased tissue while avoiding healthy tissue.
It is desired to combine the diagnostic spatial specificity of MR imaging with radiotherapy beam focus technology to provide more accurate treatment of diseased tissue while reducing the damage of healthy tissue. By combining real time imaging and radiation therapy, radiotherapy beam shaping may be performed in real time, compensating for not only daily changes in anatomy but also body movements such as breathing which occur during the treatment procedure.
In operation, a radiotherapy beam may be rotated around a patient to deposit a focused dose of radiation at the target area (i.e., diseased tissue) while sparing the healthy tissue. Combining radiation therapy with MR imaging requires that the radiotherapy beam reach a patient who is enclosed with an MR imager and scanner. Furthermore, the radiation beam should pass through the MR imager or scanner in a controlled and known manner so that the magnitude and location of energy delivered by the radiotherapy beam can be accurately controlled.
In general, the most accurate MR imagers or scanners use high magnetic fields produced by superconducting magnets which usually are composed of thick superconducting wire windings, thin metallic shells and a large cryogenic bath (e.g., liquid helium) disposed in a cryostat.
A radiotherapy beam is attenuated when it passes through matter such as metals or even liquid helium in a cryostat of the MR imager or scanner. If the attenuation or loss is held constant over time and angular position, it is possible to compensate or adjust for the loss so as to accurately control the magnitude and location of energy delivered by the radiotherapy beam.
However, during maintenance and operation of the superconducting magnet system of an MR imager or scanner, it is often the case that some amount of cryogenic fluid (e.g., liquid helium) boils off and therefore the level changes, thereby changing the attenuation of the radiotherapy beam. Furthermore, since the cryostat is typically not completely filled with liquid helium, the amount of liquid helium varies as a function of position within the cryostat, and the amount or volume of liquid helium through which the radiotherapy beam must pass may also be a function of angular position. As a result, attenuation of the radiotherapy beam is also a function of angular position. Thus it may be difficult to accurately control the amount of radiation energy delivered to a target area of interest by the radiotherapy beam to be constant, and particularly to be constant at various angular positions.
One aspect of the present invention can provide an apparatus, comprising a radiation source configured to generate a radiotherapy beam and a magnetic resonance imager. The magnetic resonance imager can include a cryostat. The cryostat can comprise: an inner chamber, and a vacuum region substantially enclosing the inner chamber. The inner chamber can comprise: first and second annular sections separated and spaced apart from each other along a first direction, and a third annular section extending in the first direction between the first and second annular sections and connecting the first and second annular sections to each other. An internal width of the third annular section in a plane perpendicular to the first direction can be less than an internal width of the first annular section and an internal width of the second annular section. The radiotherapy beam can be configured to pass through the third annular section of the cryostat
In some embodiments, the radiation source can comprise a linear accelerator.
In some embodiments, the radiation source can comprise a multileaf collimator.
In some embodiments, the apparatus can further include superconducting coils disposed in the first and second annular sections. The superconducting coils can include at least a pair of first semiconductor coils and a pair of second semiconductor coils, wherein the first superconducting coils can be disposed closer than the second superconducting coils to the third annular section, and wherein a diameter of each of the first superconducting coils can be greater than a diameter of each of the second superconducting coils.
In some embodiments, the radiotherapy beam can be configured to pass between the pair of first semiconductor coils.
In some embodiments, the first and second annular sections can have disposed therein corresponding first and second annular volumes of a cryogenic fluid, the third annular section can have disposed therein a third annular volume of the cryogenic fluid, and an annular depth of the third annular volume in the plane perpendicular to the first direction can be less than an annular depth of the first annular volume and an annular depth of the second annular volume.
In some embodiments, the apparatus can include a tubular structure extending in the first direction between the first and second annular sections.
Another aspect of the invention can provide a chamber for a cryostat. The chamber can include first and second annular sections separated and spaced apart from each other along a first direction, and a third annular section extending in the first direction between the first and second annular sections and connecting the first and second annular sections to each other. The first and second annular sections can define corresponding first and second internal volumes, the third annular section can define a third internal volume, and the third internal volume can be substantially less than the first internal volume and substantially less than the second internal volume.
In some embodiments, the first and second annular sections can have disposed therein corresponding first and second annular volumes of a cryogenic fluid, the third annular section can have disposed therein a third annular volume of the cryogenic fluid, and an average annular depth of the third volume in the plane perpendicular to the first direction can be less than an average annular depth of the first volume and an average annular depth of the second volume.
In some embodiments, the cryogenic fluid can comprise liquid helium.
In some embodiments, the cryogenic fluid can comprise gaseous helium.
In some embodiments, the first internal volume and the second internal volume each can be ten times the third internal volume.
In some embodiments, the first internal volume and the second internal volume each can be one hundred times the third internal volume.
In some embodiments, the chamber can include superconducting coils disposed in the first and second annular sections. The superconducting coils can include at least a first semiconductor coil and a second semiconductor coil, wherein the first superconducting coil can be disposed closer than the second superconducting coil to the third annular section, and wherein a diameter of the first superconducting coil can be greater than a diameter of the second superconducting coil.
In some embodiments, the internal width of the first annular section and the internal width of the second annular section each can be more than ten times the internal width of the third annular section.
In some embodiments, the internal width of the first annular section and the internal width of the second annular section each can be more than thirty times the internal width of the third annular section.
Yet another aspect of the invention can provide a chamber for a cryostat. The chamber can comprise first and second annular sections separated and spaced apart from each other along a first direction, and a third annular section extending in the first direction between the first and second annular sections and connecting the first and second sections to each other. The first and second annular sections can have disposed therein corresponding first and second annular volumes of a cryogenic fluid. The third annular section can have a third annular volume of the cryogenic fluid disposed therein. The an average annular depth of the third annular volume in a plane perpendicular to the first direction can be less than the average annular depth of the first annular volume and the average annular depth of the second annular volume.
In some embodiments, the chamber can include superconducting coils disposed in the first and second annular sections. The superconducting coils can include at least a first semiconductor coil and a second semiconductor coil, wherein the first superconducting coil is disposed closer than the second superconducting coil to the third annular section, and wherein a diameter of the first superconducting coil is greater than a diameter of the second superconducting coil.
In some embodiments, an internal width of the first annular section in the plane perpendicular to the first direction and an internal width of the second annular section in the plane perpendicular to the first direction each can be more than ten times an internal width of the third annular section in the plane perpendicular to the first direction.
In some embodiments, an internal width of the first annular section in the plane perpendicular to the first direction and an internal width of the second annular section in the plane perpendicular to the first direction each can be more than thirty times an internal width of the third annular section in the plane perpendicular to the first direction.